Numerical modelling of thermally induced regional and local scale flows in MacKenzie Basin, New Zealand

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Thesis Discipline

Environmental Sciences

Degree Grantor

University of Canterbury

Degree Level

Doctoral

Degree Name

Doctor of Philosophy

Atmospheric flows that result from surface heating and cooling in complex, mountainous terrain encompass many scales. These flows are induced by horizontal thermal gradients in the atmosphere associated with topographic relief, and are most pronounced when synoptic pressure gradients are weak. At the small-scale end of the spectrum there are slope and along-valley winds, and at the larger scale there are plain-to-basin and plain-to-plateau flows.
The intrusion of a recurring plain-to-basin wind system named the Canterbury Plains Breeze (CPB) into the MacKenzie Basin and Lake Tekapo region is described using both observational data and results from a numerical model. Observational data from a surface monitoring network designed to investigate local flows in the Lake Tekapo area show that the CPB has an important influence on the wind regime in this region. An atmospheric mesoscale numerical model was utilized to investigate the origin and forcing mechanisms for this flow.
The mesoscale model was able to successfully simulate the CPB for a case study day using realistic synoptic scale winds. To isolate the major forcing mechanisms for this flow, additional idealized two- and three-dimensional numerical experiments were conducted. The idealized three-dimensional runs showed that the CPB is a not a sea breeze intrusion into the MacKenzie Basin as was previously thought, and could be generated by orography alone.
The two-dimensional runs showed that although the sea breeze outside the basin does not have a direct impact on the CPB, it can influence the current's intensity by suppressing the mixed layer growth outside the basin, which increases the horizontal temperature gradient that forces the CPB. Other surface effects, such as soil moisture content and land use also seem to affect the characteristic features of the plain-to-basin flow.
In addition, two high-resolution simulations were performed to investigate the interaction of the CPB with locally generated thermal flows around Lake Tekapo, where the surface monitoring network was established. Results show that early in the afternoon the CPB flows into the region as gap winds through the saddles in the Two Thumb Range, dominating the local winds. These gap winds are localized in nature, and the model runs suggest that the monitoring network was not dense enough to adequately describe the surface flow field around the lake.
It is evident from this research that regional circulation systems, such as the CPB, could transport air pollutants significant distances from the coastal plains, over complex terrain into the relatively pristine environment of the South Island's mountain basins. A knowledge of the physical forcing mechanisms involved in such flows can therefore have important practical applications for air quality issues.